The present invention relates to a lattice tower crane having a main boom which is partitioned into separable elements of lattice construction, and having in addition a head part which is (or could be) secured to the tip of the main boom, the head is expepcted to have at least one pulley or sheave for guiding the lifting cable.
Cranes of the type mentioned above are known, but it has to be observed that in case of lifting bulky loads under a condition in which the main boom has an oblique orientation, the load when suspended from the cables must not hit the boom. This condition can be observed for a given lift height of the load, for example, through appropriately extending the main boom. But of course due to the lever effect, such extension of the main boom is a very costly aspect, and any particular assembly or disassembly is cumbersome and expensive. Moreover, for a given system the supportability of the crane is rendered more limited.
German printed patent application No. 2,807,415 suggests to attach an angled supplemental boom to the tip of the principal or main boom. This supplemental boom does in fact displace the first deflection pulley or sheave for the load carrying cable from the axis of the principal boom, using the load as point of reference. This way even bulky loads can be lifted, even if the main boom is relatively short, but the configuration of the supplemental boom makes sure that even a bulky load will not hit the main boom. However, there is an inherent disadvantage in that the load carrying capability of the supplemental boom reduces the capability of the crane as a whole, unless of course, under very extensive expanditure, the supplemental boom is further reinforced over and beyond the particular configuration as shown in this printed patent application. Also, of course, the assembly procedure is a costly and time consuming aspect of this approach.
For many years it is known to provide the tip of the principal boom with a so-called hammer head serving the same purpose, and, quite analogous to the aforementioned supplemental boom, effectively displaces the first deflection sheave or pulley for the cable from the axis of the principal or main boom. Also, a hammer head will normally reduce the load carrying capability of the crane to a lesser extent than a supplemental boom, as mentioned above. The known hammer heads, however, require a very extensive assembly procedure, and of course the principal boom must be equipped right from the beginning with suitable attachment equipment to which the hammer head can be connected and secured. This of course poses particular problems in the case if such a hammer head is to be attached to the main boom of a crane when this crane was not originally designed for such attaching operation. In this case then one really has to provide constructive modifications of the main boom which is actually quite undesirable in principle. Moreover, the known hammer heads of the type having a high load carrying capability are quite bulky, which means that storage and transport to the respective construction site requires a very large volume in terms of transport and storage capacity. Thus the hammer head is not only expensive to be constructed, but its very use is an expensive procedure and undertaking.